Allow unspecified max allocatable bitrate in VideoSendStreamImpl

If we have no video the can be enabled, based on the VideoEncoderConfig
(e.g. the max bitrate is <= 0 or all simulcast streams are
`active=false`) - let the max bitrate be reported as 0 for the sake of
bandwidth allocation (MediaStreamAllocationConfig).

Previous to this CL, the above conditions would result in an arbitrary
value of 10Mbps to use as the max allocation limit. When creating a peer
connection with audio and video configured but the video disabled/muted,
that would result in the bandwidth estimator trying to send packet
trains to probe up to 10Mbps, potentially impacting the network and
disturbing audio even though we have no need for such as high BWE.

Bug: webrtc:494350649
Change-Id: Id6353fd93c6170610a790c084d75f3b3c5c3ee97
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449100
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47220}